The present invention generally relates to the field of passivating micro-electronic devices and, more specifically, to a multi-layer passivation barrier for a superconducting element.
Increasingly, microelectronic devices are being designed and manufactured to incorporate superconducting elements in order to take advantage of the special properties of the superconducting materials from which those elements are made. Superconducting elements, particularly those made of oxide superconducting materials, however, are subject to degradation over time due to the indiffusion of impurities from outside the superconducting element and outdiffusion of oxygen from within the superconducting material. This in- and outdiffusion alters a superconducting material""s critical properties, i.e., critical temperature (Tc), critical current density (Jc) and critical magnetic field (Hc), which can destroy its superconducting state under desired operating conditions.
Generally, microelectronic superconductor devices include superconducting elements which each comprise one or more thin film layers of an oxide superconducting material, such as Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O, Yxe2x80x94Baxe2x80x94Cuxe2x80x94O, Tlxe2x80x94Baxe2x80x94Caxe2x80x94Cuxe2x80x94O, Hgxe2x80x94Baxe2x80x94Caxe2x80x94Cuxe2x80x94O or any other superconductor oxide. Each superconducting layer is typically adjacent other layers, such as substrate, insulator and buffer layers, which also comprise the superconducting device. These adjacent layers often contain chemical elements, such as silicon and nickel, that can diffuse into the superconducting element and destroy its superconducting state. In addition, the layers adjacent to each superconducting layer may allow chemical elements from other layers and/or impurities located outside the device, e.g., moisture, salts, alkali metals and the like, to diffuse into and destroy the superconducting state of the superconducting material. Moreover, one or more of the adjacent layers may have a large enough oxygen diffusion constant such that they will allow oxygen to outdiffuse from the superconducting material at an unacceptably high rate that destroys the superconducting state of the superconducting material.
Over the years, a number of barriers have been proposed and/or used to protect, or passivate, oxide superconducting elements from the indiffusion of impurities and outdiffusion of oxygen that are so detrimental to the superconducting state of the superconducting material. However, each of these barriers has at least one shortcoming.
U.S. Pat. No. 4,965,244 to Weaver et al., discloses a passivation layer made of CaF2, which is applied to the surface of a high-temperature superconducting element. U.S. Pat. No. 5,196,379, also to Weaver et al., discloses a method of depositing an oxide passivation layer onto a superconducting element, where the passivation layer is an oxide of Al, Bi, Si, or Alxe2x80x94W. The passivation layers disclosed in the two Weaver et al. patents, however, are not good barriers to outdiffusion of oxygen from the superconducting material. Moreover, these layers contain chemical elements, such as Si, that are known to degrade the superconductive properties of many superconducting materials.
U.S. Pat. No. 5,411,938 to Wu et al. discloses a glass layer for protecting a superconducting element from moisture and other environmental substances that are detrimental to its superconducting properties. Similar to the passivation layers disclosed in the Weaver et al. patents, the glass layer of Wu et al. is not a good diffusion barrier to outdiffusion of oxygen from the superconducting material.
U.S. Pat. Nos. 5,114,910 and 5,272,133 to Josefowicz et al. each disclose a two-layer passivation barrier consisting of a Group II element, e.g., Mg, Ca, Ba, and Sr, oxide layer deposited onto an oxide superconducting element and a polymer layer applied to the Group II oxide layer. The Group II oxide layer must be deposited as an amorphous film and does not block oxygen and/or moisture from reaching the superconducting element. In addition, it is believed that certain Group II elements may diffuse out of the corresponding oxide layer into the superconducting element and therefore may be detrimental to the superconducting material over a period of time. Moreover, the polymer layer is not a good barrier to oxygen and is not compatible with back-end-of-line processing, which frequently is performed at temperatures that would destroy the polymer layer.
U.S. Pat. No. 5,866,195 to Lemelson discloses using a diamond film as an insulating layer. High-quality diamond is typically processed at high temperatures that would cause oxygen outdiffusion from a superconducting material, thus lowering its critical temperature. In addition, carbon from the processing of the diamond layer can be detrimental to superconductor devices integrated with superconductor devices. Moreover, further processing of the diamond film at temperatures above 500xc2x0 C. in the presence of oxygen will reduce it to carbon dioxide, thus preventing a post-deposition oxygen anneal.
U.S. Pat. No. 5,480,861 to Tanaka et al. discloses a noble metal layer disposed between a thin film insulating layer and a thin film oxide superconducting element. The noble metal layer must be deposited on the superconducting element as a mono-layer in order for the insulating layer to be epitaxial. If the insulating layer were not epitaxial, the resulting reduced crystallinity would be detrimental to the properties of that layer. The noble metal layer is deposited while the temperature of the superconducting element is 700xc2x0 C. Such a high temperature causes oxygen to outdiffuse from the superconducting material, lowering its critical temperature. In addition, the noble metal layer can act as an electrical short in some circumstances.
The present invention is directed to a passivation barrier for an oxygen-containing material having a physical property. The passivation barrier comprises a first layer made of a nonpolymer material and having a first surface and a second surface. The non-polymer material comprises a constituent element that changes the physical property of the oxygen-containing material when the constituent element diffuses into the oxygen-containing material. The barrier further comprises a second layer made of a non-conductive material and having a first surface and a second surface. The first surface of the second layer confronts the second surface of the first layer. The second surface of the second layer confronts the oxygen-containing material. The second layer is a barrier to diffusion of the constituent element from the first surface of the second layer to the second surface of the second layer.
The present invention is also directed to a method of passivating an oxygen-containing material. The method includes providing a first material containing oxygen and having a physical property. Also provided is a second material containing a constituent chemical element that changes the physical property of the first material when the constituent chemical element diffuses into the first material. The second material is a non-polymer. Further provided is a third material that is a barrier to diffusion of the constituent element. The third material is deposited onto at least a portion of the first material to form a buffering layer. The second material is deposited onto at least a portion of the buffering layer to form a passivating layer.